From: "Gene McCalmont" 
Subject: RE: [drakelist] L4-PS R12 burned - .825 ohm 2W resistor on pin 6
of cable harness
Organization: RowdyHD Inc.


"Gene McCalmont"  made an utterence to the
drakelist gang
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Hello Jason;
From your email, it appears that one of your 3-500z tubes has "bit the
dust". It would be difficult to say what caused the problem, but these
tubes, while robust to a fault, are not indestructible and do have a
definable life cycle. It is entirely normal for the tube plates to have a
dull reddish glow while the amp is keyed, however, from your description, I
would say you are correct that one tube was under full load while the other
was not. While color descriptions are mostly subjective, an orangey glow
means the remaining tube was pulling a lot of current when observed.
Depending on how long this condition existed, it too is probably exhausted.

There is practically no way an average amateur can check a 3-500z unless the
tube is put into service and observed. Fortunately, you had enough presence
of mind to observe the tubes while switching off the power. Bravo! This is
the way I would proceed:

1. You will, of course, have to open the PS to replace R12. If at all
possible, replace R12 with an exact value 2w carbon resistor. If
unavailable, get as close as you can using only what is available in a 2
watt configuration metal oxide composition resistor. Remember, R12 is kind
of like a thermal fuse so any condition that causes excessive current draw
exceeding its 2w dissipation rating, will blow the fuse. Metal oxide
resistors will be slower to "give it up" than carbon, so carbon is the
choice. The value of R12 is important and fortunately, it is a commonly
manufactured part.
2. Observe the other PS components paying close attention to the filter
capacitors, bleeder resistors and all three dropping resistors. These
components may need attention as well. 
3. Go to the bottom of the RF deck and observe the components and socket
around the tube that was not conducting. Look for obvious problems such as
swollen/cracked capacitors and resistors, burnt wire etc. At this point, I
would not start wholesale replacing components. It is likely that any
component failure at this point was catastrophic and should be easily
discovered.
4. It is most likely that the one tube you observed had reached its end of
life and will need to be replaced. When you remove the tube, mark it with a
felt tip marker. What caused it to fail, having not discovered any obvious
failed components on the chassis underside, will be difficult to determine.
At this point, you have several options. Replace that one tube with another
USED tube and get on with life, or replace BOTH 3-500z tubes with fresh
examples.

Although this would be a lot of work, the most optimal approach, since the
amp is already down for the count, would be to freshen the PS with new
capacitors and diodes, and a few other components, do a really good cleaning
of the amp, service the fan motor, replace any obviously defective under
chassis components at the RF deck, plunk in some new Taylor 3-500z tubes
from RF Supply and Rock and Roll. You will have an awesome amp, capable of
1,500watts out, that will last you another 20+ years.

Good luck and please keep me informed.
Gene

W5DDW
Gene McCalmont
270 Oak Ridge Ln.
Argyle Texas 76226

-----Original Message-----
From: Jason Buchanan [mailto:jsb{at}digistar.com] 
Sent: Monday, December 06, 2004 6:42 AM
To: gene{at}pwhome.com
Subject: Re: [drakelist] L4-PS R12 burned - .825 ohm 2W resistor on pin 6 of
cable harness


Thank you VERY much for your EXCELLENT advice - I will do some checking
inside the PS today.  I did notice that one tube was dark while the other
was red, almost orange, when this happened.  I noticed a small pop sound
coming from the power supply and when I turned my head to look at the supply
and start reaching for the power switch I saw that one tube appeared to be

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